1. Field of Invention
This invention relates to the coating of the surface of a metal with an oxide material to improve its characteristics, which include, but are not limited to, abrasion resistance, heat resistance, corrosion resistance, and electrical resistance.
2. Description of Prior Art
Metals, particularly aluminum and its alloys, have been coated for many years with a thin layer of oxide in order to improve the characteristics of the material, particularly its tolerance to abrasion, oxidation, corrosion and thermal effects. Aluminum and its alloys are soft and chemically active, and so an oxide coating can greatly enhance the applicability of these metals. However, oxide coatings used commercially are porous and do not provide good corrosion protection, and the hardness and thermal properties are limited.
A common means for coating aluminum and its alloys with oxide is an electrolysis process called anodizing. The metal sample is typically placed in an electrolyte solution of 15 percent sulfuric acid, in water, and a positive direct-current voltage, typically 15 to 40 volts, is applied between the aluminum sample and a second electrode. The electrical current that flows in the electrolyte solution is typically about 2 amperes per square decimeter of the metal sample surface.
A new electrolysis process has been developed in Russia that forms greatly improved oxide coatings on aluminum and its alloys, as well as on other metals. For the electrolyte, the process generally uses an alkaline, rather than an acid. A typical electrolyte solution is 2 gram/liter of potassium hydroxide in water. The tank that holds the electrolyte solution is usually stainless steel. An alternating voltage is applied between the metal sample being coated and an inert non-reactive electrode, which is usually the stainless steel tank. The peak voltage is much greater than that used in anodizing. A typical average current per half cycle of the alternating voltage is 10 ampere per square decimeter of the surface of the metal sample.
This process has been implemented using (a) an alternating voltage source of 50 hertz frequency and typically 380 volts rms, provided by a transformer, (b) a capacitor to limit the electric current, and (c) thyristors to control the current flow. When current flows through the tank, a series electric circuit is closed from the transformer output voltage, through the capacitor, through a back-to-back pair of thyristors (called series thyristors), and through the tank electrolysis circuit, which is the electrical conduction path in the electrolyte solution between the metal sample and the stainless steel tank. A back-to-back pair of thyristors, called shunt thyristors, is placed in parallel with the tank electrolysis circuit. (A thyristor is also called a silicon controlled rectifier, abbreviated SCR.)
The series thyristors are excited during every electrical cycle in which the process operates. The shunt thyristors control the flow of current through the tank. When the shunt thyristors are excited, no current flows through the tank. When electric current flows in the electrolyte solution from the metal sample to the tank, it is called anodic current; and when it flows from the tank to the metal sample, it is called cathodic current. If the shunt thyristor having its cathode electrically connected to the tank is appropriately excited during a cycle, no anodic current flows through the tank during that cycle. If the other shunt thyristor is appropriately excited, no cathodic current flows through the tank during that cycle.
Commercially available thyristor driver circuits practically always apply a narrow voltage pulse to the thyristor control electrode, the pulse being very short relative to the period of the alternating current prime power. The pulse makes the voltage of the control electrode positive relative to the cathode for a very short interval of time.
This conventional thyristor driver approach has been used in the prior art implementations of the above described electrolysis process for coating metal with an oxide. It has serious deficiencies. It is extremely difficult, and possibly impractical, to apply the excitation voltage pulse at the optimum time. If it is applied when the thyristor is back biased, the thyristor is not turned on. If it is applied when the thyristor is strongly forward biased, a large current spike is generated, because of the capacitance in the power circuit. Current spikes degrade the quality of the ceramic coating, and generate undesirable electromagnetic interference.